A hard/tough act to follow
Definition 1 of 1
Pro Tip 1/3Impressive Predecessor
Used when someone did a job so well that it seems difficult for the next person to be as good. 
After the award-winning director left, everyone knew she'd be a tough act to follow.
Pro Tip 2/3Successor's Challenge
Highlights the daunting task faced by someone taking over from a greatly admired figure. 
The new coach has big shoes to fill; his predecessor is a hard act to follow.
Pro Tip 3/3Performance Anxiety
Can imply the pressure felt by someone who fears they can't meet high expectations. 
Being the next vocalist after her is a tough act to follow; I'm nervous.